Blender to Unity: Model Rigging Issue
If you want to apply the pose as rest pose and the model is already skinned, you need to apply the Armature modifier first, then apply the pose, then add a new Armature modifier from the modifier menu.

You can also probably join both the armatures together (CTRL-J) and parent the bow on a bone level.

But if you wanna have swap-able weapons, then the common practice is to have a dedicated weapon bone and copy it's global matrix to the weapon of choice in engine.